This invention relates to the detection and quantitative measurement of the radioactive isotope of radon, .sup.222 Rn, as encountered in air in habitats such as in typical and public buildings, and particularly it relates to a passive detection system in which the radon at the test site is adsorbed into an adsorbent such as activated charcoal.
.sup.222 Rn is a radioactive decay product of .sup.238 U which occurs naturally in the earth's crust and especially in granite rocks. .sup.222 Rn is often referred to simply as radon and that terminology will be employed here. That is, radon, as herein used, is defined to mean the specific isotope .sup.222 Rn which has a half-life of 3.82 days, decaying predominantly to the isotope .sup.218 Po with the emission of an alpha particle of 5.49 MeV of energy.
Radon is the heaviest of the inert gasses, the end of the series beginning with helium and neon. When produced, it has the properties and the lifetime to diffuse out of the minerals in which it forms and becomes a constitutent of the air we breathe. Techniques for its collection and measurement date from its discovery in 1900. Today, private and public actions to understand and alleviate the problems posed by unacceptable levels of radon require accurate measurements of the radon concentration in buildings.
Radon accounts for most of the naturally occurring ionizing radiation burden to the general population. But, because radon itself is insert, it is not considered a health hazard. Harmful effects result primarily from the decay radiations from the progeny of the radon, all of which are chemically very active. There are five sequential decays which occur in the first few hours following the decay of .sup.222 Ra. The immediate daughter of .sup.222 Ra is .sup.218 Po which t ransmutes in 3.05 minutes into .sup.214 Pb by emitting a 6.0 MeV alpha particle. .sup.214 Pb decays in turn with a half-life of 26.8 minutes, with the emission of an electron and, generally, a gamma ray, to an isotope of bismuth, .sup.214 Bi, which itself decays to .sup.214 Po in 19.8 minutes by emitting an electron and, generally, a gamma ray. Finally, .sup.214 Po decays in 164 microseconds by emitting a 7.687 MeV alpha particle. In summary, a sequence of short-lived transmutations takes place following each radon decay, and each step yields easily detectable radiation. In a matter of hours, three alpha particles, two electrons, and about two gamma rays are emitted for every radon decay.
The dangers posed by radon have led to guidelines for the permissible levels of radon in air. In domestic environments the U.S. Environmental Protection Agency recommends that an average yearly concentration should not exceed 4 picoCuries per liter of air; henceforth, 4 pCi/l. Once Curie represents a radioactivity level of 3.7.times.10.sup.10 disintegrations per second. One pCi equals 3.7.times.10.sup.-2 disintegrations per second or 2.2 disintegrations per minute. The EPA criterion is, therefore, 4.times.2.2=8.8 disintegrations of radon per minute per liter of air. The total number of disintegration per minute per liter of air containing 4 pCi/l of radon is then 5.times.8.8=44 dpm. The importance of the radon problem may be judged by the fact that the EPA estimates that as many as 20% of the homes in the United States, between 15 and 20 million homes, may have radon levels which exceed 4 pCi/l.
The preponderance of radon measurements of air carried out in recent years have used passive methods in which the radon-bearing air is allowed to diffuse into activated charcoal; charcoal has been found to be the most effective adsorbent of radon from the air since it was first used about 1910; other adsorbents may be used and more effective ones may yet be found but we will use activated charcoal as the standard example. We note, however, that the invention does not depend on activated charcoal; any radon adsorbent can be used providing that the radon can be desorbed into a liquid scintillation cocktail.
After the charcoal has been saturated with radon (which takes from 1 to 7 days depending on the design of the passive detector) the radon concentration may be determined by one of several sophisticated techniques.
The best-known method for determining the radon, adsorbed by the activated charcoal, measures the gamma radiation emitted in the decay of two of the progeny of radon. Liquid scintillation methods, used in this invention, detect all the charged particles emitted in the decay of radon and its progeny. The LS technique has not been widely accepted though it has been known for years and is acknowledged to be one of the most sensitive techniques for determining low levels of radon. The main reason for the poor reception of LS methods is the complexity of the desorption procedure as described in the literature. Since the thrust of this invention is to simplify that procedure so that the technique will gain wider acceptance, we first review the relative advantages and disadvantages of the gamma ray and liquid scintillation techniques for determining the radon concentration in charcoal, and then show how the present invention obviates the principal disadvantage of the LS technique.
Activated charcoal is a highly effective adsorbent of the radon in the air; one gram at room temperature pulls the radon from about 4 liters. The charcoal, placed in a porous container, accumulates the radon out of the air until saturation is reached. The container is then sealed and sent to the appropriate laboratory for measurement of the radon content; portable measuring systems are practical but are not now being utilized. The gamma ray technique determines the radon concentration from the intensity of the gamma rays emitted in the third and fourth links in the decay chain of radon. That is, the rate of gamma rays emitted in the decay of .sup.214 Po to .sup.214 Bi and the subsequent decay of .sup.214 Bi to .sup.214 Po can be related in a straightforward manner to the rate of decay of radon atoms in the charcoal sample. The method has many advantages. First, large amounts of charcoal can be used, giving a strong signal, since the gamma rays can penetrate many centimeters of charcoal without being attenuated. Second, the penetrating power of the gamma rays makes practical the use of a metal container for the carbon which need not be disturbed for measurement; that is, the measurement can be carried out by simply placing the unopened charcoal-containing canister in front of the gamma ray detector. Third, the canisters are robust and can be recycled for multiple use without undue difficulty. Fourth, the gamma ray counting apparatus is commercially available.
However, the gamma-ray counting method has serious disadvantages. First, only two of the five links in the radon chain are detected so that the greater part of the radon signal is ignored. Second, the detection of gamma rays is not very efficient; a 3" Diam.times.3" detector (the one generally used) counts only about 10% of the gamma rays emitted by the radon daughters in these charcoal canisters. Third, the low efficiency of detection necessitates using 25 to 100 grams of charcoal. Thus the ability to use large amounts of charcoal is a practical requirement. The charcoal-filled metal canisters are bulky and the costly to ship quickly, as radon testing requires. Shipping costs can make up a substantial cost of the radon test. Fourth, moisture problems, which can give apparent radon values which are two to three time smaller than the true values, cannot be alleviated in practice by the use of desiccants which must be the same weight as the charcoal to be effective. Fifth, while counting facilities utilizing personal computers are commercial available, there are not automated conveyer feeds for the metal canisters. They must be loaded into place by hand for each measurement. Sixth, the background counts in the large Nal detectors are substantial and make it difficult to measure low radon levels in a short time. The importance of this point is emphasized by the values quoted in the standard reference on the technique of measuring radon concentrations in charcoal through the counting of the gamma radiation. In that work, Bernard I. Cohen and Richard Nason, Health Physics, Vol. 50 1986, pages 457-463, give a detailed description of the method now in common use. On page 462, they quote the true counts and the background counts for their system after a 25 gram charcoal canister was exposed to 1 pCi/liter. The overall result for a 30 minute counting time is 260 true counts compared to a background of about 1600 counts in the same time interval. The system described by Cohen and Nasan evolved over several years of development and it must be assumed that their signal to noise results are close to the optimum for a practical system. It should be noted that increasing the size of the detector will give greater detection efficiency but will also increase the background counts and the ratio of signal to background will not be greatly affected.
Each of the disadvantages of gamma ray detection are offset if the radon in the charcoal is measured by liquid scintillation counter (LSC) techniques. First, the LSC technique detects all the charged particles which result from a radon decay. Thus it detects 5 signals rather than 2 for each decay of radon. Second, the liquid scintillation (LS) technique has close to 100% efficiency for detection of these high energy charged particles. This gives the LSC method a further advantage of almost an order of magnitude greater sensitivity. Third, the background counts of 15 to 25 counts per minute are typically ten times lower for LSC than for gamma ray detection, primarily because the liquid scintillant has much smaller volume than the Nal(Tl). These three advantages give the LSC technique a 100 fold advantage over the gamma ray detection technique, so that LSC is more effective for radon detection using a two gram adsorbent than is a 25 gram adsorbent using gamma ray detection. Fourth, the small weight of adsorbent gives the method a further advantage, since it makes practical the use of desiccants of the same volume to obviate humidity problems. The small size of the LS detectors also significantly reduces the cost of mailing and handling. Fifth, the LS counters for small (&lt;20 cc) samples, commonly used in biomedical areas, have automated counting and sample handling. A typical LS counter can be loaded with 200 more samples for computer controlled measurement. There are some 10,000 LS counters in the field.
To emphasize the inherent advantages of LS over gamma ray techniques, we compare the time needed to obtain the same statistical accuracy in the two methods when the activated charcoal is exposed to a radon atmosphere of 1 pCi/l, a typical value found in homes. At this radon level, Cohen and Nasen, using gamma rays, report a signal to background ratio of 260:1600 for a 30 minute counting time of their 25 g of charcoal; the statistical uncertainty is 23%. Using the liquid scintillation technique and a 2 g charcoal detector, we obtain 55 counts per minute from the radon versus 25 counts per minute from the background. In one minute of counting we obtain the value of the radon concentration with the same statistical uncertainty that took 30 minutes with gamma ray counting. One will appreciate that there are commercial advantages to reducing the counting time of an expensive instrument by a factor of 30 while using 12 time less charcoal.
The single advantage enjoyed by gamma ray detection over LS detection is that of simplicity of the counting procedure. The only manipulation of the aluminum charcoal canister is in feeding it on and off the gamma ray detector. The LS method, as described by Prichard and Marien in the well known paper:
Desorption of radon from activated Carbon into a Liquid Scintillator, Analytic Chemistry 55, 155-157, 1983, involves complex steps. Until now, this has been the factor which has deterred potential uses of the LSC techniques and has been the decisive advantage favoring the gamma ray method. The elution procedure is the central problem addressed in the Prichard-Marien paper. Since we will contrast the simplicity of the present invention with the complexity of the Prichard-Marien technique, we quote their procedure in pertinent part below. PA1 The prepared carbon was exposed to radon gas (and) transferred to an 80-mL separatory funnel connected to a 60-mL flask containing a known volume of reagent grade toluene. When the stopcock was opened, the toluene flowed down onto the carbon, producing an exothermic outgassing reaction. The evolved gas passed upward through the toluene, affording an opportunity for any radon in the gas to be transferred to the liquid. After a few seconds of gentle shaking the toluene passed entirely into the funnel and the stopcock was closed . . . After a wait of at least 2 h for desorption, the funnel was shaken and inverted, a syringe attached below the stopcock. the free liquid portion removed. The toluene [together with 1 or 2 mL of concentrated fluor solution] was transferred to a 22-mL glass liquid scintillation vial for . . . counting after a 3-h delay for the ingrowth of radon daughters.
The Pritchard/Marien technique involves the following steps: 1. The radonbearing carbon is transferred to a separatory funnel. 2. A known volume of toluene is flowed into the carbon. 3. The funnel is shaken for a few seconds and the stopcock closed. 4. After two hours desorption time the funnel is shaken again and inverted. 5. 1 to 2 ml of concentrated fluor (the scintillant) solution is added to the toluene. 6. The cocktail is transferred to a 22 ml glass LS vial for counting which is down 3 hours after the transfer.